Aldax Moulds and Mouldmaking Supplies

How To Cast Concrete Into Rubber Concrete Molds

This a brief overview of the methods of concrete casting into rubber concrete molds to get you up and running. For more in depth information ask for our e-Book on the subject.
The commonly used term concrete casting as it applies here refers to a mixture of sand, Portland cement and water. In contrast, building concrete more commonly contains additional amounts of an aggregate such as blue metal or gravel. Our mix is designed to be fluid and decorative and capable of being poured into rubber concrete molds rather than structurally strong and load bearing.
The larger rubber concret molds as used for garden decoration must be well supported due to the weight of the sand and cement mixture. A fiberglass back-up mold must be used with most of the larger rubber concrete molds to encase and thus support the latex mold, to ensure the casting has a perfect shape. (unsupported latex is elastic and will stretch out of shape).
An alternative with the smaller molds is to support the mold upside down in a sand box. The sand should be moist so that it stays in shape but does not distort the mold. Another method is suspend the mold in a bucket of water or sawdust.
When using a sand box, half fill the mold and then squeeze it to push the cement/sand mix into every corner of the mold before setting back in the sand box, packing the damp sand around the mold to just under the top. Continue filling the mold with the cement mix.
We make recommendations with our garden rubber concrete molds as to whether a back-up mould is essential, advisable or not necessary. Back-up molds can be made to order for all rubber concrete molds, ring for details.

" A plastic mixing tub or a wheelbarrow for larger projects
" A shovel, spade or trowel
" Mold release. Use a vegetable oil (machine or car oil affects the latex)
" Dust mask, rubber gloves, plastic sheeting

Mix 2 parts of sharp sand to one part of fresh Portland cement then add water and stir till you get a thick creamy mix.
Alternatively, purchase a bag of ready mixed sand and cement from your local hardware store and proceed as above.
The mix must be pourable, not too watery or too dry.

Fill the mold about three quarters full and then squeeze to get rid of all the air bubbles. Prod the mix in the mold with a stick in an up and down action to bring any bubbles to the surface and ensure all sections of the mold are fully covered with the mix. Continue filling and the prodding action till the mold is full.
To ensure all the air bubbles have gone gently lift and tap the mold or the sand box on the bench for 10 to 20 seconds. Professionals would be using a vibrating table to do the same thing.

Cover with a plastic sheet and set aside out of the sun in a cool location for 24 hours to harden. Extra time may be necessary in winter.
At the end of this period remove the mold from the back-up mold or the sand box, lubricate the outside of the mold with warm water and soap or dish washing liquid and then pull the mold from the casting.

To allow the casting to fully cure and reach maximum strength cover it with cloth or hessian and keep it damp with a sprinkler for one week. Keep it out of the sun during this period, in a cool damp spot.

By Stan Alderson
Copyright ŠAldax Enterprises Pty Ltd 2010


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